17:15 〜 19:15
[MIS17-P05] Raman elastic barometry of coesite-in-kyanite: Elastic anisotropy simulation for refining UHP metamorphic metamorphism
キーワード：超高圧変成作用、コース石、藍晶石、異方性弾性モデリング、ラマン地質圧力計
Ultrahigh-pressure (UHP) metamorphic rocks, formed at depths exceeding 100 km and later exhumed, provide crucial insights into deep subduction dynamics and crustal recycling. Over the past four decades, the discovery of coesite- and diamond-bearing crustal metamorphic rocks has advanced our understanding of crust-mantle interactions at collisional margins. However, precisely determining peak metamorphic pressures within the coesite or diamond stability fields remains challenging. Conventional thermodynamic methods often fail above ~3 GPa due to disequilibrium, limited mineral assemblages, and uncertainties in H2O activity. To mitigate these limitations, Raman-based elastic barometry offers a promising alternative by directly inferring pressures from the residual stress in UHP mineral inclusions.
In this study, we develop a coesite-in-kyanite Raman elastic barometer using UHP whiteschist from the Dora-Maira Massif, Italian Alps (Takeda, 2025 Master's thesis). Raman spectroscopy of coesite inclusions (4–45 µm) in kyanite (~250–500 µm) reveals that the pressure-sensitive 521 cm–1 band shifts up to 526.8 cm–1, corresponding to a residual pressure of ~1.97 GPa. By focusing on small (<25 µm) coesite inclusions that lack retrogressed quartz shells and visible fracturing, we obtained a reliable residual pressure estimate of ~1.49 GPa. However, kyanite's lower symmetry (triclinic) amplifies strain heterogeneities due to crystallographic misalignment, making the evaluation of relative crystallographic orientation (RCO) critical. To interpret these residual stresses comprehensively, we integrated Raman measurements with elastic anisotropy modeling. Our modeling is based on elastic calculations optimized for anisotropy and inclusion morphology. Specifically, we simulate the effects of RCO and inclusion shape on residual stress in coesite-bearing kyanite using a numerical approach following Furukawa and Tsujimori (2024), based on the method of Moulinec and Suquet (1998).
Under a plausible RCO scenario, our approach yields entrapment conditions of ~4.6 ± 0.1 GPa at 730°C, consistent with previous constraints on UHP metamorphism in the Dora-Maira Massif. Additionally, Raman imaging, processed via spectral intensity mapping, reveals anisotropic stress fields in the host kyanite, which are crucial for verifying model assumptions. The observed anisotropic residual stresses in kyanite align well with predictions from elastic modeling. These combined approaches demonstrate that Raman-based elastic barometry can achieve uncertainties of ±0.1–0.2 GPa, rivaling or surpassing conventional methods. Ultimately, this study provides a reliable method for constraining peak pressures in UHP mineral inclusions, enhancing our understanding of continental subduction processes at convergent margins and improving the accuracy of geodynamic models.
Furukawa, T., Tsujimori, T., 2024, The Cahn–Hilliard model of coherent lamellar microstructure: application to alkali feldspar, Contributions to Mineralogy and Petrology, v. 179, 91, https://doi.org/10.1007/s00410-024-02169-2.
Moulinec, H., Suquet, P., 1998, A numerical method for computing the overall response of nonlinear composites with complex microstructure. Computer Methods in Applied Mechanics and Engineering, v. 157, p. 69–94, https://doi.org/10.1016/S0045-7825(97)00218-1
Takeda, N., 2025, Raman spectroscopic study of coesite inclusions in kyanite: Elastic anisotropy modeling and application to Dora-Maira UHP whiteschist. Tohoku University, Master's thesis, 182 p.
In this study, we develop a coesite-in-kyanite Raman elastic barometer using UHP whiteschist from the Dora-Maira Massif, Italian Alps (Takeda, 2025 Master's thesis). Raman spectroscopy of coesite inclusions (4–45 µm) in kyanite (~250–500 µm) reveals that the pressure-sensitive 521 cm–1 band shifts up to 526.8 cm–1, corresponding to a residual pressure of ~1.97 GPa. By focusing on small (<25 µm) coesite inclusions that lack retrogressed quartz shells and visible fracturing, we obtained a reliable residual pressure estimate of ~1.49 GPa. However, kyanite's lower symmetry (triclinic) amplifies strain heterogeneities due to crystallographic misalignment, making the evaluation of relative crystallographic orientation (RCO) critical. To interpret these residual stresses comprehensively, we integrated Raman measurements with elastic anisotropy modeling. Our modeling is based on elastic calculations optimized for anisotropy and inclusion morphology. Specifically, we simulate the effects of RCO and inclusion shape on residual stress in coesite-bearing kyanite using a numerical approach following Furukawa and Tsujimori (2024), based on the method of Moulinec and Suquet (1998).
Under a plausible RCO scenario, our approach yields entrapment conditions of ~4.6 ± 0.1 GPa at 730°C, consistent with previous constraints on UHP metamorphism in the Dora-Maira Massif. Additionally, Raman imaging, processed via spectral intensity mapping, reveals anisotropic stress fields in the host kyanite, which are crucial for verifying model assumptions. The observed anisotropic residual stresses in kyanite align well with predictions from elastic modeling. These combined approaches demonstrate that Raman-based elastic barometry can achieve uncertainties of ±0.1–0.2 GPa, rivaling or surpassing conventional methods. Ultimately, this study provides a reliable method for constraining peak pressures in UHP mineral inclusions, enhancing our understanding of continental subduction processes at convergent margins and improving the accuracy of geodynamic models.
Furukawa, T., Tsujimori, T., 2024, The Cahn–Hilliard model of coherent lamellar microstructure: application to alkali feldspar, Contributions to Mineralogy and Petrology, v. 179, 91, https://doi.org/10.1007/s00410-024-02169-2.
Moulinec, H., Suquet, P., 1998, A numerical method for computing the overall response of nonlinear composites with complex microstructure. Computer Methods in Applied Mechanics and Engineering, v. 157, p. 69–94, https://doi.org/10.1016/S0045-7825(97)00218-1
Takeda, N., 2025, Raman spectroscopic study of coesite inclusions in kyanite: Elastic anisotropy modeling and application to Dora-Maira UHP whiteschist. Tohoku University, Master's thesis, 182 p.